Color television receivers use the 3.58 MHz chrominance burst signal, generated by the transmitter, to synchronize and otherwise condition the receiver's locally-generated chrominance reference oscillator. Because only approximately eight cycles of the burst signal are provided on the "back-porch" of the horizontal synchronizing pulse, it is desirable to "gate" the burst signal so that only it, and not spurious signals or noise components, are allowed to reach the chrominance processing stages.
Historically this has been done by using the horizontal flyback pulse as the burst gating signal, relying on its coincidence with the burst period. Such systems have proved less than ideal because, inter alia, the horizontal flyback pulse typically has a width of approximately 13 microseconds, whereas the period of the burst signal is in the neighborhood of 2 microseconds. Furthermore, because of errors in the free-running frequency of the receiver's horizontal oscillator, there may be as much as a .+-.4 microseconds difference between the occurrence of the synchronizing pulses and the flyback pulses. As a result, the burst gate pulsewidth is necessarily made wider than the burst duration, to the detriment of the chrominance control characteristics of the receiver. This is especially evident when the first microsecond or so of an active line is allowed to pass through the burst gate. The reference oscillator will then possess an error component related to the signal that has erroneously passed through the burst gate. The color content of the picture will vary with the error component.
A more accurate burst gating pulse can be derived from the trailing edge of the horizontal sync pulse. In one form, the horizontal sync pulses can be differentiated and used to open the burst gate. However, because a differentiating circuit necessarily has the characteristics of a high-pass filter, noise pulses received with the composite video signal will also open the burst gate, thereby introducing erroneous information into the chrominance reference channel.